1. Field of the Invention
The invention relates to a honeycomb structure having a honeycomb body of electrically conductive material being electrically divided by slots and/or electrically insulating layers in such a way that an electric current can flow along a winding path through the body.
Such bodies are known from Published International Application WO 89/10470, corresponding to U.S. Pat. No. 5,146,743; Published International Application WO 89/10471, corresponding to U.S. Pat. No. 5,322,672 and to co-pending U.S. application Ser. No. 08/051,348, filed Apr. 27, 1993; or Published European Application No. 0 452 125 A2, corresponding to U.S. Pat. No. 5,063,029, for instance.
In Published European Application No. 0 452 125 A2, corresponding to U.S. Pat. No. 5,063,029, which is the point of departure of the present invention, many different possibilities for dividing a honeycomb body by means of slots are described. However, it has been demonstrated that an electrical current which is compelled to take a meandering path always has a tendency to find the shortest path, in other words the path with the least electrical resistance. A substantial proportion of the current therefore flows within a very small region near the slots or layers that limit the electrical conductivity. Typically, a honeycomb body of the above-described construction through which current flows in a meandering path will not be heated uniformly but instead has especially hot spots on the insides of the curves in the current path. That may even lead to red-hot heating or melting or at least to softening of the body, which impairs its stability and service life. Particularly when such a honeycomb body is used as an electrically heatable catalytic converter in a motor vehicle, that is of decisive significance, because high strains occur there and only limited electrical energy is available, which must be made use of as optimally as possible.
Published European Application No. 0 452 125 A2, corresponding to U.S. Pat. No. 5,063,029, mentions various possibilities with which the electrical resistance in different portions of the body should be varied. Those include varied wall thicknesses of the individual honeycombs, a different number of honeycombs per unit of cross-sectional area, or a different axial length of individual cell walls. However, in principle such provisions are not suited to attaining a more homogeneous distribution of the current in an electrically conductive honeycomb body. The reason for that is that the specific electrical resistance of the material always remains the same, so that by providing more or less material, the resistance in that region can be varied, but not the specific current density per unit of material present. For instance, if the wall thickness is increased in a region in which especially strong currents flow, then the resistance decreases there and even stronger currents flow, so that the thicker material becomes heated up just as much as thinner material. Conversely, if the wall thickness or the number of cells is decreased, then while a lesser current flows, nevertheless it heats the lesser quantity of existing material just as strongly as before. The same is logically true for axially variable lengths of the cell walls as well.